A blockchain is a distributed ledger that can be used to record transactions and reduce the likelihood that recorded transactions are altered retroactively. Generally, a blockchain can be implemented within a peer-to-peer network. Computing devices participating in the peer-to-peer network can each include a copy of the blockchain and broadcast transactions to other computing devices participating in the peer-to-peer network.
The blockchain itself can include a chain of blocks that are linked together cryptographically, with each block in the chain storing one or more transactions and the number of blocks in the chain growing over time. For example, a transaction can include a cryptocurrency payment, execution of a smart contract, submission of an online vote, and/or any other type of exchange or event. The blocks can be linked together using a cryptographic hash. For example, each block can include a cryptographic hash of a previous block in the chain. Because each block includes a cryptographic hash of a previous block in the chain, a transaction stored in one block cannot be altered without all subsequent blocks being altered as well. The likelihood that all subsequent blocks being altered is low given that such an alteration would require approval from a majority of the computing devices or participants participating in the blockchain.